EXHALATION FROM LEAVES. 77 



thermometer having risen to a maximum of 27° 7" [82° Fahr,], the 

 minimum having been only 10° 7* [57° Fahr.] ; the wind on this day 

 and the preceding had been from the north or north-east. Seventy- 

 two hoin*s after the introduction of the oak branch into the water the 

 vase had again lost 140 grammes [5 oz.] of its weight. The weather 

 had been as lovely as the day before, the minimum of the temperature 

 having been at 12° 5" [54°] and the maximum 25'' [77°]. Thus the 

 ■weight of water evaporated by the branch was, at the end of the first 

 day, 510 grammes, at the the end of the second, 810, and at the end of 

 the third, 950 [30 ounces]. The experiment was not cai-ried further. 

 In the first place, because several of the leaves were withered and 

 incapable of performing their functions ; secondly, because the water 

 in the jug was beginning to be affected by being mixed with the sub- 

 stances which always are deposited by plants when placed in water. 



" If we believe that all the leafy parts of a tree will act, as regards 

 the faculty of transpiration, like the leaves of the above-mentioned 

 branch : in other words, if we suppose that the quantities of water 

 expired by the entire tree were in like proportion, we arrive at the 

 astounding result that an oak like the one described will, in a summer 

 day, cause the evaporation of more than 2000 killogrammes of water, 

 more than two cubic metres, [or 440 gallons]. 



" We are not decided in regard to the value of our experiment, and 

 we see very weU that our deductions are not free from objections ; but 

 it must be allowed that supposing that every one-half or one-quarter 

 of the estimated quantity be omitted, the quantity must greatly 

 exceed what might have been expected 



" A forest should produce on the soil which it covers the effect of 

 a large umbrella pierced in many places, it may be, but, nevertheless, 

 arresting and imbibing a certain quantity of rain, of which it deprives 

 the neighbouring soil. So long as the leaves are imperfectly soaked, 

 no moisture reaches the ground ; and it can be easily imagined that 

 a moderate continuous rain would be entirely absorbed by natural 

 evaporation, and in effect the humidity of the air, even in damp 

 weather, never reaches the maximum of the hygrometer in the lower 

 atmospherical strata, and evaporation is never entirely suspended. 



" If we further add to these causes of exhaustion that there is little 

 dew deposited under trees, it can be believed that the ground of a 

 forest is less favoured as regards the quantity of moisture received 

 from the heavens than that which is exposed to the open sky. 



" And, nevertheless, it is an accepted fact, and that not without 

 good reason, that the neighbourhood of forests is cold and damp. 



